None of the DLC was on the disc when it shipped. The game modes are a small download because they are rules. There is some new content, but it's obviously a different type of content compared to a map. Also, the recent patch added a bunch of missing support/engine groundwork for both DLCs.
We are limited in the number of matchmaking buckets we can use due to Live. We left a bunch of buckets available for the DLC, but perhaps we made too many team oriented buckets. We'll evaluate changing/modifying that as we get feedback. FWIW, since the DLC modes are generally more team-based than the default game, we skewed the new hoppers to be team-focused as well.
Lead Designer, Halo Wars